Electric discharge apparatus



y 4, 1963 c. B. STADUM ETAL 3,089,948

ELECTRIC DISCHARGE APPARATUS Filed Feb. 21, 1952 1:5 Sheets-Sheet 1 Non-Repeat WITNESSES: SEQUENCE TIMER INVENTORS Clarence B. Srodum,Edword C. Hortwig d g-' and Hubert W. Van Ness. 6% BY [fi dim ZgL W ATTORNEY May 14, 1963 c. B. STADUM ETAL 3,089,948

ELECTRIC DISCHARGE APPARATUS Filed Feb. 21, 1952 13 Sheets-Sheet, 2

V WELDER Starting Relay 223 Welding Transformer I REVERSING 37 I UNIT POST HEAT UNIT Bios 6th On 9th Off 5th On 9th Off POSTHEAT 381 NETWORK WITNESSES: 263 INVENTORS 5 c g- I Clarence B.Stodum,Edword C.Hortwig ong Hubert W. Van Ness.

ATTORNEY May 14, 1963 c. B. STADUM ETAL 3,089,948

ELECTRIC DISCHARGE APPARATUS Filed Feb. 21, 1952 13 Sheets-Sheet 3 39' 59FTI FT4 BiOS 5 ZSOI I Weld Post Hem Relay w PosrHeuf Lending H.C. Network WITNESES: INV ENTORS FigJQ ClarenceBSfodum,EdwordC.Hariwig andHuberf W.Von Ness.

ATTORN EY y 1963 c. B. STADUM ETAL 3,089,948

ELECTRIC DISCHARGE APPARATUS Filed Feb. 21, 1952 13 Sheets-Sheet 4 i-i i 59 POWER SUPPLY UNIT 69 FTS 39 ALB) [192 T 2 21g\2 3soz-wi zssa 5 woe-g Following H.C. Network WITNESSES: \HEAT CONTROL UNIT INVENTORs ClorenceBsiodum Edword C.Hortwig 5%7yw4? Fig. ID. ofld tlubert W. Van Ness.

1 61944 ag QAWMflM C ATTORNEY y 14, 3 c. B. STADUM ETAL 3,089,948

ELECTRIC DISCHARGE APPARATUS Filed Fgb. 21, 1952 13 Sheets-Sheet 5 Following H.C. l6!

PostHeot WlTN a sss; I NVENTOR s I I Clarence B.S1odum Edward C.Hortwig l ancM-idbert w.VonNess.

ATTORNEY May 14, 1963 Filed Feb. 21, 1952 C. B. STADUM ETAL ELECTRIC DISCHARGE APPARATUS l3 Sheets-Sheet 6 Fig.|F. POWER SUPPLY UNIT F|GS.|C, ID8 |E. K3 AL4 224 Kl 12 1-3 T l w 1-4 1-5 1-6 M s g E :25;

I l IKZ: P A I AL3) 222 I WELDER +N 4 F|G.|B. l i

l ALI) /L3 SEQUENCE Ali) L12 TIMER REVERSING UNIT U) F|G.|A. FIG

Coble- POST HEAT UNIT HEAT CONTROL UNIT FIGS.IC,ID& IE.

WITNESSES: INVENTORS Clarence B.S1odum,Edwcrd C.Hor1wig ong Hubert w.von Ness.

ATTORNEY May 14, 1963 c. B. STADUM ETAL ELECTRIC DISCHARGE APPARATUS l3 Sheets-Sheet 8 Filed Feb. 21, 1952 I'll I ll/, 23 E m; cozEo l Em: Foal o I 7, V/ lI/ 20 4 w bm N H :95: H gll otuJ moth. 01H COZEOHI 27/ 71? 32 o 4|1 23 $2 m; cotEo I mot TH co co l III wE m co co l l I I l I ll 3.930 6 25 0: 55m I l l llot 92 cot o l I l I II 25 E I 5 l E T C AAAAAAAAAAAWAAnn m; E U III/[Ill E m; 5: 59 I Q wE w; co co l l I l II N E m; EEE ifi V v II amwlivifi s I||| moi w; :68 IIIIII ll Secondory Current |||1 mS m; co cS I| Secondary Current Curve for Invention Curve for Prior Art. Disclosedv Fig.3.

INVENTORS Clarence B.Stodum,Edword C.Har1*wig cnd Hubert W. Van Ness.

ATTORNEY May 14, 1963 c. B. STADUM ETAL ELECTRIC DISCHARGE APPARATUS 13 Sheets-Sheet 9 Filed Feb. 21, 1952 (I L G M A m S N N U E R r e m m0 3 d & L I 9 en Wm4 T e .w s w m R sh 89 RR Starting Reloy E E 5 6 52; muzwncwm zea INVENTORS Clarence B.Srodum,Edword C.Hortwig WITNESSES:

im 2 k Fig.4.

and Hubert W.VonNes 75 2M ATTORNEY y 1953 c. B. STADUM ETAL 3,089,948

ELECTRIC DISCHARGE APPARATUS Filed Feb. 21, 1952 13 Sheets-Sheet 10 K AL4 WELDER K7 Weld I Responsive Welding Relay Transformer REVERSING UNIT ALI SEQUENCE TIMER-Shown in Fi IA.

384 399 am On 2nd Off 9 ff lOIh On 372 40 AT? HEAT CONTROL UNIT 0nd POWER SUPPLY UNIT Shown in Fi S.IC ID OndIE.

SIR 8% 407 as? 383 Post Heat All-5 H87 5 Network WITNESSES: 263 R INVENTORS R Make before break ClorenceB.Sfodum,Edword C.HorIwiq RRI- Break before make. n H er w.von Nes i/ l m Fig.5. 7 7

ATTORNEY y 4, 1963 c. B. STADUM ETAL 3,089,948

ELECTRIC DISCHARGE APPARATUS Filed Feb. 21, 1952 l5 Sheets-Sheet 11 5 WITNESSES: 6 ELNVENZOR'St t Clarence BSTudum, word .Hor wlg Zfi on ubert W. Van Ness.

ATTORNEY y 4, 1963 c. B. STADUM ETAL 3,089,948

ELECTRIC DISCHARGE APPARATUS Filed Feb. 21, 1952 13 Sheets-Sheet 12 WITNESSES:

ATTORNEY INVENTORS Clarence B.Stodum,Edword C-HOrTwig y 14, 1963 c. B. STADUM ETAL 3,089,948

ELECTRIC DISCHARGE APPARATUS Filed Feb. 21, 1952 13 Sheets-Sheet 13 Fig.8.

Welding Transformer CONTROL Resilient Micro-Switch MS I MI I M2 0 i l 'Ii I Fig.9. MI I w WITNESSES: INVENTORS Clarence B.Stodum,Edword 'C.Hcr1wig ongyubert W. Van Ness.

ATTORNEY United States Patent O ELECTRIC DEQHARGE APPARATUS Clarence B. Stadurn, Snyder, Hubert W. Van Ness,

Buiiaio, and Edward C. Hartwig, Lancaster, N.Y., asignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 21, 1952, Ser. No. 272,818 20 Claims. {CL 219-414) Our invention relates to electric discharge apparatus and has particular relation to apparatus for controlling the supply of current from a polyphase source of alternating current to a load such as an electric welder.

Patent 2,508,467 to J. R. Parsons and Edward C. Hartwig, and Patent 2,619,591 to J. R. Parsons, are illustrative of the prior art relating to our invention. In the welding system disclosed in the Parsons et al. patent, the welding transformer has three primaries and a single secondary. Power is supplied directly to these primaries from the buses of a polyphase source through three pairs of ignitrons connected in anti-parallel between the buses and the primaries. With each of these six ignitrons, a firing tube is associated. Each firing tube is controlled from a heat-control tube connected in a heat-control circuit. The conductivity of the six heat-control tubes is controlled from a sequence timer. The Parsons application discloses a similar system. Such systems have the advantage that the ignitrons may be connected directly to the buses of a three-bus commercial power supply. Thus, the neutral-tapped supply transformer customarily used with such apparatus is avoided.

On the whole, the type of welding system disclosed in the Parsons et al. patent and in the Parsons application has proved satisfactory for many purposes. But, we have found in our work with such a system that it has certain disadvantages. The transformer is uneconomical in its copper requirements. Because the transformer includes three primaries, each operating for one-third of the time of operation of the transformer, it requires an excessively large quantity of copper. The control circuit is highly complex and because of this complexity is costly and imposes high installation and maintenance costs. In addition, the many interrelated electronic components which this circuit includes confront servicing personnel with difliculties in locating a source of defective operation. in addition, in spite of its high cost, such a system is incapable of delivering sufiicient power for the welding of heavier gauges of such materials as aluminum, brass, bronze, and the like.

it is, accordingly, an object of our invention to provide apparatus of relatively simple and low-cost structure having low installation and maintenance cost for controlling the supply of power from a polyphase source to a welder.

Another object of our invention is to provide relatively simple and low-cost electric discharge apparatus which is adapted to be interposed directly between the buses of a commercial polyphase source which does not have a neutral and a welding transformer for controlling the supply of power to the welding transformer.

Still another object of our invention is to use a singlephase load transformer, such as a welding transformer, for example, having minimum copper requirements, with discharge apparatus for controlling the supply of power from a polyphase source to the transformer, which discharge apparatus shall be adapted to be connected directly and not through a supply transformer having a neutral tap, to the buses of the source.

A further object of our invention is to provide an improved welding system particularly suitable for the welding of heavy gauge metals and alloys such as aluminum, brass, bronze and the like.

A still further object of our invention is to provide an improved welding system particularly suitable for welding together the ends of rods composed of such metals as aluminum.

An incidental object of our invention is to provide an improved converter for converting the power from a commercial polyphase source to single phase at a frequency lower than that of the source.

Another incidental object of our invention is to provide an improved heat-control circuit for a low-frequency welder adapted to be energized from a polyphase source.

A further incidental object of our invention is to provide an improved relay circuit particularly suitable for controlling an electric welder.

An ancillary object of our invention is to provide a novel sequence timer particularly adapted to operate with control apparatus in accordance with our invention.

Another ancillary object of our invention is to provide a novel heat-control network particularly suitable for producing welding current of such wave form. that it has a post-heating function as well as a welding function.

In accordance with our invention, we provide a system including a welding transformer having only a single primary which is supplied through a current-reversing mechanism from a converter directly connected to the buses of a polyphase source. The reversing mechanism performs the function of periodically reversing the current flowing through the primary, and the converter need be capable of delivering current of only one polarity to the mechanism and may be of simple, low-cost structure.

The converter comprises a plurality of pairs of main discharge devices, the devices of each pair being connected directly to a supply bus and, through the reversing mechanism, to the terminals of the primary. The discharge devices are so connected that at any instant current may flow in one direction between two buses through associated discharge devices and the primary, current flowing away from the positive bus through one of the latter devices and returning to the negative bus through the other. With the reversing mechanism set in one position, the discharge devices are rendered conductive in succession, starting with two selected devices, to conduct current of one polarity during a predetermined interval which may be substantially longer than a period of the commercial supply. At the end of the interval, the discharge devices remain non-conductive for a predetermined quiescent time interval. The reversing mechanism is operated during this quiescent zero-current interval, and during the succeeding conductive interval, current of opposite polarity flows through the primary.

Where welds are to be produced in rapid succession, as in a seam welder, the discharge devices are rendered conductive in succession, and the reversing mechanism is operated repeatedly at a predetermined periodicity. The current flowing through the primary is then, in effect, lowfrequency alternating current having a period equal to the time which elapses between the beginnings of successive conductive intervals of the same polarity. Sometimes, independent spot welds are produced at substantial intervals. Under such circumstances, succeeding spot welds are produced with current of the opposite polarity if the reversing mechanism is operated during the quiescent intervals. In either case, magnetic saturation of the welding transformer is avoided by the current reversals.

While our invention involves a number of features, there are several which are of particular importance. Several of these we shall now describe.

One important feature involves the operation of the reversing contactor. if the reversing contactor is actuated while weiding current flows, its life will be materially reduced. It is important, then, that provisions be made for assuring that the contactor is not actuated while weiding current is flowing. We, therefore, provide a s 'stem 3 including a sequence tirner which cooperates with the reversing contactor to actuate it during the intervals the electrodes are pressed against the material being welded, known as the squeeze intervals.

A portion of each squeeze interval is consumed in actuating the contactor and the squeeze interval must be sufliciently long for this purpose. in accordance with another feature of our invention, this disadvantage is avoided by conditioning the contactor to operate during the weld interval and operating it during the hold interval.

Another important feature of our invention is the provision of a direct-current reversing contactor. Since the contactor is operated repeatedly, it must be so designed and constructed as to withstand the repeated shocks. While a sturdy A.C. contactor serves reasonably satisfactOrily, we have found that a DC. contactor is more satisfactory. A D.C. contactor is operated by current maintained continuously at the same level, while an A.C. contactor is operated by current which varies repeatedly between zero and a maximum value. To achieve the same operation, the amplitude of the current supplied to an A.C. contactor must be substantially higher than the mag nitude of the current for a DC. contactor. The coils of a. DC. contactor are then smaller than those of an A.C. contactor, and the contacts of the former are not actuated so sharply as the contacts of the latter. A DC. contactor is thus better able to withstand the wear of repeated actuation than an A.C. contactor.

Preferably, the DC. contactor is of type MM sold by Westinghouse Electric Corporation (see page 2.3226, Temporary Industrial Control Engineering landbook) which is capable of 15,000,000 repeated operations without failure. This contactor has two sets of contacts and a pair of operating coils for operating these contacts. The contacts are positively interlocked and are so arranged that one or the other set is normally closed while the remaining set is open. Positive control of the reversing of the current flow through the primary of the welding transformer is thus achieved.

Still another important feature of our invention involves the connection of main discharge devices to the welding transformer. These devices are so connected that at any time two devices conduct current through the primary of the transformer in series, one conducting from one supply bus to the primary and the other from the primary to another supply bus. When the welding current is initiated, it is then essential that two such discharge devices be rendered conductive simultaneously. In accordance with a further feature of our invention, we provide a heat-control circuit for accomplishing this purpose.

In certain situations, particularly where the materials to be welded are of substantially different thicknesses, it is desirable that current conducted during succeeding intervals be of a selected polarity only. In accordance with still another feature of our invention, we provide control means which provides not only that current of alternating polarity is conducted through the primary but also that the discharge devices conduct current through the primary of the welding transformer either in one direction or the other, depending on the desires of the operator. Thus, an operator welding materials of substantially different thickness may set the control so that for a time, current of one polarity is transmitted through the primary. With this setting, the welding is effected with the thicker material, while the thinner material is in one position relative to the welding electrodes. Thereafter, the setting may be changed so that the current flow through the primary of the welding transformer is reversed, and the position of the materials relative to the welding electrodes may be reversed. With this control, saturation may also be avoided.

The cooperation of the discharge apparatus and the reversing mechanism according to our invention provides a high degree of flexibility in the operation of the apparatus. At the same time, the discharge devices required and their associated components are maintained at a minimum, and the system is relatively simple, reqiuring a minimum of installation and maintenance.

In accordance with a further important feature of our invention, the main discharge devices are rendered conductive in succession at intervals equal to 360/2n, where n is the number of phases of the supply. Where a system embodying our invention is supplied from the usual threephase supply, the discharge devices are thus rendered conductive at intervals of /6 of a period. Initially, two discharge devices, one connected to each terminal of the primary, are rendered conductive; then discharge devices connected to one terminal and the other of the primary are rendered conductive alternately in succession, and after each new discharge device begins to conduct, one of the devices which has been conducting becomes non-conducting. During such operation, there is a short time interval, called the commutation period, during which these devices are conductive.

We have found that with the discharge devices thus rendered conductive at intervals of Ms of a period, sound welds can be produced with relatively heavy gauge metals such as aluminum, bronze, and brass. The success in the welding of such metals appears to arise from the fact that the current which our invention is capable of supplying to the primary is substantially higher than the current available in a system of the same rating of the prior art type.

During each commutation period defined above, when three devices are conductive, a large current, which we may call a commutation fluctuation, flows through the welding transformer, and a sharp decrease occurs in the voltage of the power conductors of the system. In available sequence timers, the weld time is timed out by a thyratron which has impressed in its control circuit a ripple voltage derived from the supply for determining the duration of the weld time. We have realized that such sequence timers are not suitable for our system because the commutation fluctuations entirely destroy the ripple wave form. In accordance with still another aspect of our invention, we have provided a sequence timer with a ripple-free control circuit for the .thyraitron which times out the weld time.

While the control of the discharge devices as outlined above is to be preferred, the discharge devices may, in accordance with the broad aspects of our invention, be rendered conductive differently. For example, the discharge devices connected to the opposite terminals of the primary may be rendered conductive simultaneously in pairs at intervals of 360/n of a period where n is the number of phases. Thus, Where the supply is of the three-phase type, the discharge devices may be rendered conductive at intervals of /3 of a period of the supply. In apparatus of this latter type, the current available for welding would not be as high as in apparatus of the type discussed above.

The features of our invention that we consider novel are set forth in the appended claims. The invention, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIGURES 1A, 1B, 1C, 1D and 1E together are a circuit diagram of an embodiment of our invention;

FIG. 1F is a block diagram which will facilitate understanding of FIGS. 1A to IE;

FIGS. 2 and 3 are graphs illustrating the operation of the apparatus shown in FIG. 1;

FIG. 4 is a circuit diagram showing the essential features of a modification of our invention;

FIG. 5 is a circuit diagram showing a further modification of our invention;

FIG. 6 is a plan view in front elevation of a directcurrent reversing contactor which preferably is used as one feature of our invention;

FIG. 7 is a view in side elevation with a section broken away of this contactor;

FIG. 8 is a diagrammatic view illustrating a welding system embodying our invention for the welding of the ends of rods; and

FIG. 9 is a diagrammatic view illustrating steps in a welding operation performed with the system shown in FIG. 8.

Description for FIGS. 1A to 1E The Welding system shown in FIGS. 1A to IE consists of a Welder, a Reversing Unit, a Power Supply Unit, a Heat-Control Unit, a Post-Heat Unit, and 3. Sequence Timer. The relationship between these units is shown in FIG. 1F. Power for the system is derived from the buses L1, L2, L3 of a commercial three-phase source in which are connected three reactances RX RXZ and RX3 which absorb the eifect of a short circuit or commutation fluctuations in the apparatus. These reactances may be lumped or they may be the available regulation of the supply. The power for welding is derived directly from these main buses; the power for operating the various control components is derived from auxiliary buses ALT and ALZ which are supplied from buses L3 and L2 through a transformer T.

The Welder includes welding transformer W, having a single primary P and a secondary S, a pair of welding electrodes E1 and E2 connected to S, one of which, Ell, is movable by a hydraulic cylinder C in and out of engagement with material M to be welded, a magnetically actuable valve V which controls the admission of fluid under pressure into the cylinder, and a pressure switch PS operated by a pressure relay PR which closes when adequate pressure is applied by the electrodes to the material M.

Each of the windings P and S of the transformer W may, of course, be made up of a number of sub-windings connected either in series or in parallel. The primary P is provided with four terminal conductors 11, 13, i5 and 17. The solenoid SV of the valve V is controlled from a starting relay SR having a pair of normally-open contacts 19 and 21.

The Reversing Unit includes two pairs of reversing contacts K1 and K2, K3 and K4 of reversing contactors S541 and respectively, each pair actnable by an exciting coil 23 and 25, respectively. The pair K1 and K2 is connected or adapted to be connected to two opposite terminal conductors 11 and 13, and the other pair K3 and K4 to the opposite terminal conductors and 17. The unit also includes a stepping or ratchet relay RL. This relay comprises a stepping rod 27, actuable only when the relay is energized, which actuates a ratchet wheel 25 that moves the movable contact 31 of the relay RL from one position to a succeeding position as it is actuated. 1n alternate positions of the movable contact 31 of the ratchet relay RL, it engages fixed contacts 32. and current is supplied to the exciting coil 23 or" one of the reversing contactors SKI from auxiliary buses ALll and AL2 through a normallyclosed contact K5 of the other reversing contactor 8K2. In the other positions of the movable contact 31 of the ratchet relay RL, it engages fixed contacts 34 and current is similarly supplied to the exciting coil 25 of the other reversing contactor SK2 through a normally closed contact K6 of the first reversing contactor SKI. The exciting coil 33 of the ratchet relay RL is supplied from the auxiliary buses AL1 and AL2 through a normally open contact 35 of an auxiliary relay AR. The exciting coil 37 of the latter is supplied from the auxiliary buses ALI and AL2 through the normally-open contact 21 of the starting relay SR.

The Power-supply Unit includes two sets each of three ignitrons 1-1, 1-2, 1-3, and 1-4, 1-5, 1-6. The anodes 39 of the ignitrons 1-1, 1-2 and 1-3 are connected each to a bus L3, L2 and Li, respectively, of the supply, and the corresponding cathodes 41 are connected together to an auxiliary bus ALE which is connected to one or the other terminal 11 or 17 of the primary P through the reversing contact K1 or is which happens to be closed. The cathodes 41 of the ignitrons 1-4, 1-5 and 1-6 of the other set are each connected to a bus L3, L2 and L1, respectively, and their anodes 39 are connected together to an auxiliary bus AL4- which is connected to one or the other terminals 13 or 15" of the primary P through the reversing contacts K2 or K3 which happens to be closed. With one ignitron of each set conductive and one pair of reversing contacts closed, current flows through the primary P in one direction. When one ignitron of each set is conductive, and the other pair of reversing contacts closed, current flows through the primary P in the opposite direction.

The Power-Supply Unit also includes a firing thyratron FTl, FTZ, FTS, FTd, FTS and FTd, respectively, for each of the ignitrons 1-1 to 1-5. The cathodes 4 3 of the firing thyratrons FT}, FTZ and PT? associated with the ignitrons 1-1, 1-2, 1-3, having their cathodes a l connected to the conductor ALE, are connected to the corresponding igniters 45 of the ignitrons, through current-limiting resistors 47, 4-5? and 55. and the contacts 53, 55 and 57 of a weld-no-Wcld relay RWN. The anodes 59 of these thyratrons are connected directly to the anodes 39 of the corresponding ignitro-ns 5-1, 1-2, 1-3, respectively. The cathodes 53 of the remaining firing thyratrons FT4, FTS and FT? are directly connected each to its corresponding igniter 4-5 through a current-limiting resistor 61, 633 and 6S, and the anodes 5% of these thyratrons are connected together through a common contact 6 7 of the weld-no- Weld relay RWN to the auxiliary bus ALd, in effect, then, to the anodes 3? of the ignitrons 1-4, 1-5, 1-6. It is to be noted that the anodes 59 of the thyratrons 5T4, FTS and FM may be connected through a common contact 67' because the anodes of the corresponding ignitrons 1-4, 1-5, 1-6 from which the current for the corresponding igniters 25 is derived are at a common potential; a similar connection is not suitable for the thyratrons FTl, FT2, F353 because the anodes of the latter are at different potentials. Separate contacts 53, 55, and 57 are required for the thyratrons F'Tll, PTZ, and FT3 because the cathodes 43 of these thyratrons must be connected separately to the separate igniters 45.

Between the control electrode 69 and the cathode 43 of each thyratron FTll to FT6, a surge-suppressing capacitor 71, '73, 75, '77, "I? and i i respectively is connected. Each control electrode 69 is also connected to its corresponding cathode 43 through a grid resistor 83, 85, 7, Sh, 9i, and 93, a pair of additional resistors 95, @7, 9?, Edit, m3 and 1615, and M7, 169, 111, and 117 across each of which a firing signal is impressed, and a bias voltage source 119, 12 1, 123, 125, 127 and 12%, respectively.

The Heat-Control Unit includes a leading heat control network N1 (FIG. 1C) and two following networks N2 (FIG. 1D) and N3 (FIG. 1B). The leading network N1 is associated with two of the buses L2 and L3 of the supply and controls the firing of the four ignitrons 1-1, 1-2, 1-4 and 1-5 connected to these buses. The following network N2 is associated with the buses L1 and L3 and controls the firing of the ignitrons 1-1, 1-3, 1-4 and 1-6, and the network N3 is associated with the buses L1 and L2 and similarly controls the firing of the ignitrons 1-2, 1-3, 1-5 and 1-6.

The heat-control network N1 is supplied with power from the buses L2 and L3 through a transformer T81. The following heat-control networks N2 and N3 are also each correspondingly supplied from the buses with which it is associated, through a transformer T8 2 and T83.

Each of the transformers T81, T52, T53 has a pair of secondaries 1531, 2531; i852, 2532; and 1533, 2583, each provided With terminal taps and an intermediate tap.

The leading network includes three thyratrons 1NT1, 2NT1, 3NT1. The anodes 131 of two of the thyratrons 1NT1 and 2NT1 are connected each to a terminal tap of the secondary 1881 through the primary 1P01 and ZPOI of an output transformer 1T01 and 2101. The cathodes 133 are connected together to the anode 135 of the other thyratron 3NT1', the cathode 137 of the latter is connected through an output resistor R01 to the intermediate tap of the secondary 1581. Between the control electrode 141 and 143 and the cathode 133 and 137, respectively, of each thyratron 1NT1 and 2NT1 and 3NT1, respectively, a surge-suppressing capacitor 14-5, 14 7, 149 is connected.

The other supply secondary 2551 provides potential for a phase-shift network PN1 which determines the instants in the half periods of the phase of the supply from which the leading heat-control network N1 draws its power when the thyratrons 1NT1, 1NT2 and 1NT3 are rendered conductive. The phase-shift network consists of a capacitor 151 and a variable resistor 153, connected to opposite terminal taps of the secondary 2981. The capacitor 151 and resistor 153 are interconnected through the manually adjustable networks PHW1 and PHP1 which are connected in series. One or the other of these networks is at any time during operation shunted out by a contact 157 or 155 of the weld-pcst-heat relay RWP depending on the position of this relay. The networks PHW1 and PHPl each include a rheostat 159 and 161 shunted by a resistor 163 and 165, respectively, and a variable resistor 167 and 162", respectively, in series with the shunted rheostat.

The series connection of the network PHWI and PHPI is an important feature of our invention. In analogous apparatus provided in accordance with the teachings of the prior art, such networks are connected in parallel, and one or the other is connected in the phase-shift network by contacts analogous to the contacts 157 and 155. In such systems, the phase-shift network is entirely open-circuited during the hiatus when the relay which corresponds to relay RWP is passing from one position to the other and a large transient is produced in the control circuits of the heat control thyratrons which fire or fail to fire improperly and, in turn, cause the corresponding firing thyratrons to operate improperly. In our system as just described, the resistances of the two networks PHWl and PHPI are connected in series during the hiatus when the relay RWP is operating and the transient is suppressed.

The intermediate tap of the secondary 2881 is connected to the control electrode 141 of the thyratron 1NT1 through a grid resistor 171, and to the cathode 133 of the thyratron 2NT1 through a resistor 173. The junction of the capacitor 151 and the networks PHW1 and PHPI is connected to the control electrode 141 of thyratron 2NT1, through a grid resistor 175, and to its cathode 133 through a resistor 177. The potentials impressed between the control electrode and the cathode of thyratron 1NT1 is thus in opposite phase to the potential impressed between the control electrode and cathode of thyratron ZNTI. This phase relationship corresponds to the phase relationship of the potentials impressed between the anodes and the cathodes of the respective thyratrons from the secondary 1581.

The rheostats 159 and 161 may be manually set to determine at what angle in the half periods of the supply the thyratrons 1NT1 and 2NT1 are fired. The angle of firing corresponds to the setting of rheostat 159 if relay RWP is deener gized and to the setting of rheostat 161 if relay RWP is energized. In practice, rheostat 159 is set to correspond to the welding current required and rheostat 161 to the post-heat (or annealing) current required.

Between the control electrode 143 and the cathode 137 of the third thyratron 3NT1, a grid resistor 179, a bias voltage source 181, an additional resistor 18?. which we may call a lock-out resistor, and an input network 1N1 consisting of a capacitor 183 in parallel with a resistor 185 are connected in series. The bias voltage from the source 181 is such as to maintain the thyratron 3NT1 non-conductive. So long as it is non-conductive, neither of the other tyratrons 1NT1 or 2NT1 can conduct. The thyratron 3NT1 is rendered conductive when a potential counteracting the bias is impressed across the network 1N1. This thyratron may then conduct, and the thyratrons INTI or 2NT1 in series with it are rendered conductive each in its turn at instants in the half periods of the supply determined by the setting of the phase-shift network PNl. When each of the latter thyratrons conducts, current is supplied through the associated output primary 1P01 or ZPOl of the associated output transformer 1TO1 or 2TO1.

Each of the latter transformers is provided with a pair of secondaries 1801, 55301 and 2501, 4301. One secondary 1801 is connected across the resistor 107 in the control circuit of the firing thyratron FT1 of the ignitron I-l Whose anode 39 is connected to bus L3, and the other secondary 5301 is connected across the resistor 103 in the control circuit of the firing thyratron FTS of the ignitron 1-5 whose cathode 41 is connected to bus L2. It is seen that the secondaries 1301 and 5501 are connected in the control circuits of two ignitrons L1, L5, which conduct simultaneously between bus L3 and bus L2. The secondaries 2801 and 4801 are similarly connected to ignitrons I-2 and I-4, 2801 being connected across the resistor 97 in the control circuit of the firing thyratron FTZ of ignitron I-2 and the other secondary 4501 being similarly connected across the resistor 113 in the control circuit of the firing thyratron FT4 associated with the ignitron L4. The leading network N1 thus controls the ignitrons I-1, 1-2, I-4 and I-5 with which it is associated. It is seen that the secondaries 1801, 5501 and 2801, 4501 are so connected to the control circuits of the ignitrons I1 and I-5 and I-2 and I-4, respectively, that when transformer 1T01 is supplied with current, ignitrons I-1 and I'5 are simultaneously rendered conductive and contact current through the primary P; and when transformer 2T01 is supplied with current, ignitrons I-2 and L4 are simultaneously rendered conductive and conduct current through the primary.

The following heat-control networks N2 and N3 are substantially similar to the leading network N1. Each includes a common thyratron 3NT2 and 3NT3, respectively, connected in series with a pair of thyratrons 1NT2 and 2NT2, and 1NT3 and 2NT3, respectively, a phaseshift network PN2 and PN3, respectively, including manually actuable control units PHWZ and PHPZ, and PI-IWS and PHP3 for setting the phase of the network, and output transformers 1T02 and 2T02, and 1T03 and 2T03, respectively.

The networks PHWZ and PHW3 are set for weld and are in their respective phase-shift circuits when contacts 187 and 189 of relay RWP are open and contacts 191 and 193 closed, and the networks PHPZ and PHP3 are set for post heat and are in their respective phase-shift circuits when contacts 191 and 193 of the relay RWP are open and contacts 187 and 189 c1osed. The secondaries 3302 and 4S02, and 1802 and 6502 of transformers 1T02 and 2T02, respectively, are connected to control the firing of the ignitrons I-3, I4, I-1 and I-6 corresponding to the network N2 in the same manner as the corresponding secondaries of the network N1, and the secondaries 2503, 6S03, 3803 and 5803 are similarly connected to control the firing of the ignitrons I-2, I6, I-3 and L5 corresponding to the network N3.

In addition to the above-described components which correspond to like components of network N1, networks N2 and N3, respectively, include a rectifier 195 and 197, and 199 and 29-1, and a resistor 2133 and 205, and 207 and 209, in series with each other and in parallel with a scess the anode 131 and the cathode 133 of each of the thyratrons 1NT2, 2NT2, 1NT3, 2NT3, respectively. The rectifiers 195, 197, 199, 201 respectively connected in a sense to conduct current from the anodes to the cathodes of the tyratrons 1NT2, 2NT2, 1NT3 and 2NT3, and to provide anode potentials for the thyratrons 3NT2 and 3NT3 so that the latter are conditioned to conduct even if the thyratrons 1NT2, 2NT2, '1NT3, 2NT3 are nonconductive. The resistors 203, 205, M17, 263" limit the current thus conducted to a low value and at the same time provide s'ufiicient drop across the anodes 131 and the cathodes 133 of the correspond-ing thyratrons to a sure that the latter conduct when their control potentials are proper.

The output resistor R01 of the leading network N1 is connected between the control electrode 143 and the oathode 137 of the thyratron 3NT2 of the first following network N2 through :a bias voltage source 211 and a grid resistor 213 in such a sense that when current flows through this resistor, the bias is counteracted and the common thyratron 3NT2 is rendered conductive. Thereafter at proper instants in the half periods of the potential supplied by transformer 2TO2, thyratrons 1NT2 and 2NT2 alternately conduct in series with thyratron 3NT2.

The common thyratron 3NT2 of the first following network N2 is connected to the center tap of the associated supply secondary 1882 through an output resistor R02, and this resistor is connected between the control electrode 143 and the cathode 137 of the common thyratron 3NT3 of the second following network N3 through a bias voltage source 215 and a grid resistor 217 in such a sense that when either one or the other of the thyratrons 1NT2 or 2NT2 conducts in series with thyratron 3NT2, thyratron 3NT3 is rendered conductive. Thereafter, thyratrons 1NT3 and 2NT3 alternately conduct in series with thyratron 3NT3 at present instants in selected half periods of the supply. The thyratrons of the first following network N2 are thus rendered conductive when the thyratrons of the leading network N1 conduct, and the thyratrons of the second following network N3 are rendered conductive when the thyrat-rons of the first following network N2 conduct. The heat-control networks for all the six ignitrons may thus be rendered effective to control the ignitrons by only one short signal impressed on the control electrode of the common thyratron 3NT1 of the leading network N1.

The Sequence Timer includes a plurality of main thyratrons ST, WT, HT and OT. These thyratrons ST, WT, HT and OT initiate or terminate certain functions of the apparatus and at the same time start the timing operations of the squeeze, weld, hold and off timing networks SN, WN, HN and ON, respectively.

It may be of interest to explain at this point what each of these components does. The squeeze thyratron ST prepares the apparatus for welding by causing the welding electrodes E1 and E2 to compress the material M. This operation occurs during a time interval determined by the squeeze network SN. The timing out of network SN is started by the squeeze thyratron. When the squeeze network times out, the actual welding is started by the weld thyratron WT. 'I his thyratron also starts the timing out of the weld network WN. When the network WN times out, the hold operation is started by the hold thyratron HT. During this operation, the electrodes are held in engagement with the material M until the weld solidifies. The hold :thyratron also starts the timing out of the hold network HN. When this network times out, the off operation is started by the off thyratron OT. During this operation the electrodes are disengaged from the material M and the material is set for a second weld. The oif thyratron OT starts the off interval by charging the ofi? network; the timing out of the off network is started by the off thyr-atron OT when the latter is rendered non-conductive.

The anode 219 and the cathode 221 of the squeeze thyratron ST are connected between the buses AL2 and AL1 in a circuit which extends from bus AL2 through the exciting coil 223 of the starting relay SR, the anode 219, the cathode 221, conductor 222, a start switch F8 for starting the operation to bus AL1.

The Sequence Timer includes a repeat-non-repcat switch RNR which may be set on repeat if the apparatus is to produce a series of welds repeatedly or on non-repeat if the apparatus is to produce only one weld. The apparatus will be described in detail here with the switch RNR set on repeat.

With the switch RNR set for repeat, the repeated operation of the thyratron ST is controlled from the elf network ON. This network consists of a capacitor 225 shunted by a variable resistor 227 and a fixed resistor 229 connected in series. The ofi network is connected between the control grid 231 and the cathode 221 of the squeeze thyr-atron ST through a grid resistor 235.

The off network ON is set for timing by the off thyratron OT. The anode 237 and the cathode 239 of this thyratron are connected in a circuit extending from bus AL1 through the starting switch PS, conductor 222, the off network ON, the conductor 241, anode resistor 242, the anode 237, the cathode 239, to the bus AL2.

The squeeze network SN consists of a capacitor 243 shunted by a variable resistor 245 and a fixed resistor 247 connected in series.

The anode 259 and the cathode 26d of the weld thyratron WT are connected in circuit with the input network 1N1 (FIG. 1C) to the leading network N1. This circuit extends from. one of the auxiliary buses AL2 through a normally open contact 262 of the start relay SR, the conductor 262, the network 1N1, a current-limiting resistor 261, a conductor 263, the anode 259, the cathode 260 to the other bus AL1.

It is to be noted that the anode circuit of the weld thyratron WT is maintained open at the contact 2432 of the start relay SR. Thus, until the start relay is actuated by the squeeze thyratron, the supply of current to the input network 1N1 is prevented. False operation during the warm-up time, when power is first supplied to the apparatus, is thus suppressed.

The weld network WN consists of a capacitor 265 shunted by a pair of variable resistors 267 and 269, one

. of which may be shunted out by the post-heat switch 270 when there is to be no post heat. This network controls the conductivity of the hold thyratron HT and is connected between the control grid 271 and the cathode 273 of the hold thyratron through a grid resistor 281.

The hold network HN consists of a capacitor 283 shunted by a fixed resistor 235 and a variable resistor 236 connected in series. This network controls the off thyratron OT and is connected between the control grid 287 and the cathode 239 of the thyratron OT through a grid resistor 299.

To produce the sequential operation of the squeeze, weld, hold, and off thyratrons and the corresponding networks, several intermediate operations are required. These operations are produced by auxiliary thyratrons and auxiliary networks which will now be discussed.

One auxiliary thyratron AT2 operates together with the squeeze thyratron ST to start the operation of the sequence timer once the squeeze thyratron is energized. This thyratron charges an auxiliary network AN2 consisting of a capacitor 307 shunted by a resistor 309. This network is connected between the control electrode 311 and the cathode 3 15 of the thyratron AT3 through a grid resistor 313. The thyratron AT2 is connected in a circuit extending from bus AL2 through the network AN2, a current-limiting resistor 315, the anode 317 and cathode 31? of the thyratron AT2, conductor 222, the starting switch PS, to the bus AL1.

The thyratron AT2 is controlled from the off network ON. The control grid 321 of the thyratron AT2 is connected to the network ON through a grid resistor 325.

The squeeze network SN consists of a capacitor 243 shunted by a variable resistor 245 and a fixed resistor 247 connected in series. The anode-cathode circuit of the thyratron AT3 extends from one auxiliary bus ALl through the squeeze network SN, a current-limiting resistor 3 33, the anode 335, and cathode 335 of the thyratron ATS to the other auxiliary bus AL2.

The squeeze network is connected through the resistor 257 between the control grid 337 and the cathode 339 of still another auxiliary thyratron AT4, which may be called the fourth auxiliary thyratron. The thyratron AT4 is connected in a circuit extending from one auxiliary bus ALE through another resistance-capacitor network AN3, the pressure switch PS of the welder, a current-limiting resistor 34%, the anode 341 and cathode 339 of the thyratron AT4 to the other bus ALll. The network AN3 consists of a capacitor 343 shunted by a resistor 345 and the secondary HS3 of a heater transformer HT3, which introduces a ripple voltage in the network. The heater transformer HTS supplies heating power for the cathode 335 of a third auxiliary thyratron AT3. The network ANS is connected through a grid resistor 349 between the control grid 351 and the cathode 347 of a fifth auxiliary thyratron ATS. The secondary H83 introduces a ripple voltage in the control circuit of thyratron ATS, the purpose of which is explained in Patent 2,533,369 to E. C. Hartwig. The anode-cathode circuit of the fifth auxiliary thyratron AT5 is supplied from the transformer T1 and includes the weld network WN. The circuit extends from one terminal of the secondary S1 through one anode 352 and the cathode 354 of a double diode D2, a currentlimiting resistor 356, the anode 353 and cathode 347 of ATS, the weld network WN to the other terminal of the secondary S1. 7

The thyratron ATS also controls the Weld thyratron through another section 362-354 of the double diode D2. This circuit includes the auxiliary network AN4 consisting of a capacitor 355 shunted by a resistor 357 and the secondary HSW of the heater transformer HTW for the weld thyratron. This network is connected between the control grid 359 and the cathode 26% of the weld thyratron WT through the grid resistor 364. The network AN4 is connected in a circuit extending from the bus ALI through the network, the anode 362 and the cathode 354 rOf the double diode D2, the resistor 356, the anode 353 and the cathode 347 of the thyratron ATS to the bus ALZ.

The weld thyratron WT is also controlled from another auxiliary network AN5 which is in the anode-cathode circuit of the hold thyratron HT and is charged when the latter conducts. The network ANS includes a capacitor 366 shunted by a resistor 368 and is connected between the suppressor grid 370' and the cathode 260 of the weld thyratron WT through a grid resistor 372. When the network is charged, it impresses a blocking bias on the weld thyratron WT, causing it to become non-conductive in spite of the state of the auxiliary network AN4. The anode-cathode circuit of the hold thyratron HT including network ANS extends from one auxiliary bus ALI through the network ANS, a resistor 374, the anode 376 and cathode 273 of the thyratron HT to the other bus ALZ.

The network ANS is also connected through a grid resistor 363 between the control grid 365 and the cathode 367 of a sixth auxiliary thyratron AT6. The anode-cathode circuit of this thyratron AT6 is connected in series with the hold network HN in a circuit extending from one of the auxiliary buses AL2 through the hold network HN, a current-limiting resistor 369, the anode 3'71 and cathode 367 of the auxiliary thyratron AT6 to the other bus ALI.

The usual -surge-suppressing capacitors are connected between all of the grids of the main and auxiliary thyra trons which are controlled and their cathodes. These capacitors do not concern the present invention, and they are shown but not labeled.

The networks of the sequence timer have different modes of operation, depending on their functions. Each of the networks is charged through a thyratron and each becomes effective to block the conduction of one or more other thynatrons once their charging thyratrons are rendered conductive. The auxiliary networks AN2, AN3, AN4, ANS become effective to permit the thyratron or thyratrons which they control to become conductive immediately after their charging thyratrons become nonconductive. The squeeze, weld, hold, and off networks SN, WN, and HN and ON become effective to permit the thyratron or thyratrons which they control to become conductive only after an appreciable time interval predetermined by the setting of a rheostat.

The Post-Heat Unit includes the weld-post-heat relay RWP (FIG. 1C). The exciting coil 373 of this relay is connected in the anode-cathode circuit of a thyratron PT (FIG. 1B), which may be called the post-heat thyratron and which has two branch circuits. One branch circuit extends from the bus AL2 through the coil 373, the postheat switch 372, the anode 377 and cathode 379 of the thyratron PT to the bus ALI. The other branch circuit extends from the auxiliary bus AL2, normally open contact 262 of the starting relay SR, the resistor 182, a normally closed contact 332 of the weld-post-heat relay RWP, the post-heat switch 372, the anode 377 and cathode 379 of the thyratron PT to the bus ALI. The coil 373 is shunted by a resistor 381 in series with a rectifier 383 so poled as to cause the relay to drop out slowly so that, in spite of the fact that it receives only alternate half waves of current, it does not chatter.

The thyratron PT is controlled from an auxiliary thyratron AT7 through another auxiliary thyratron ATS which functions as an anode potential phase inverter for the thyratron PT. The anode circuit of the inverter thyratron AT8 is coupled to the grid circuit of the postheat thyratron PT through a transformer CT, the primary CP of which is connected to the anode 384 of the thyratron ATS through an anode resistor 386. The secondary CS of the transformer CT is connected between the grid 388 and the cathode 379 of the thyratron PT through a bias 390, which holds the latter off unless thyratron ATS is conductive, and a grid resistor 392. The secondary CS is so poled that when a half-wave pulse is transmitted through the primary CP, the trailing loop of this pulse produces the positive potential to counteract the bias 3%. This loop is in phase with the anode potential of the thyratron PT. The thyratron AT7 is energized from another auxiliary transformer T2 and includes in its anode-cathode circuit the post-heat timing network PN.

Network PN consists of a capacitor 385 shunted by a fixed resistor 337 and a variable resistor 389. This network is connected between the control grid 391 and the cathode 392 of the inverter thyratron ATS through a capacitor 395 and grid resistor 399.

Between the control grid 401 and the cathode 403 of the auxiliary thyratron AT7, a network AN6 consisting of a capacitor 405 shunted by a resistor 497 in series with the secondary H57 of the heater transformer HT7 for the thyratron AT7, is connected through a grid resistor 46?. The capacitor 405 of the network ANS is charged from thyratron AT4 when the latter is conductive in a circuit extending from bus AL2, through network AN6, a conductor 411, the resistor 340, the pressure switch PS, the anode 341, and cathode 339 of thyratron AT4 to bus ALI. When the capacitor 465 is so charged, it applies a blocking bias voltage to the thyratron AT7 which becomes non-conductive.

Operation for FIGS. 1A to IE When the power switch (not shown) for the apparatus is closed, the cathodes of all the thyratrons are heated and the various auxiliary transformers are energized. Before the welding operation is initiated, the weld-noweld relay RWN is energized by closing the switch 416 connecting its energizing coil to the buses ALl and ALE 18) and its contacts 53, 55, 57 and 67 are closed, completing the circuit between the firing thyratrons FT to FT6 and the ign iters 45 of the ignitrons 1-1 to 1-6. The ratchet relay RL will have stopped in one of its positions or the other, depending on the last operation before the apparatus when used previously was shut down. In FIG. 1B, this relay is typically shown in a position in which contactor SKJI is energized and contactor 8K2 is deenergized. It is noted that the ratchet relay RL has the human attribute of memory, in eflect, remembering the polarity of the last half cycle of current before the system was shut down.

The starting switch F3 for the apparatus is now open, the anode circuits of the squeeze thyratron ST and the auxiliary thyratron AT2 are open, and these thyratrons are non-conductive. The starting relay SR, the auxiliary relay AR, and the ratchet relay RL are deenergized. Since the second auxiliary thyratron AT2 is initially nonconductive, the capacitor 307 of the second timing net work AN2 is discharged. The auxiliary thyratron ATE; is then initially conductive, charging the capacitor 243 of the squeeze network SN. When the network SN is charged, a blocking potential is maintained on the control grid 337 of the auxiliary thyratron AT4 and the auxiliary thyratron is blocked. Because the auxiliary thyratron AT4 is non-conductive, the capacitor 343 of the network AN3 is discharged and the auxiliary thyratron ATS is conductive. Because the thyratron ATS is conductive, the capacitor 265 of the weld network WN is charged and the hold thyratron HT is non-conductive. Also, because the thyratron ATS is conductive, the capacitor 355 of network AN4 is charged and the weld thyratron WT is non-conductive. Because the hold thyratron HT is non-conductive and the capacitor ass of the network ANS is discharged, the auxiliary thyratron ATti is conductive. The capacitor 283 in the hold network HN is then charged and the off thyratron OT is nonconductive. The capacitor 225 in the otf network is then discharged and the squeeze thyratron ST and the auxiliary thyratron ATZ can conduct immediately when their anode circuits are closed by means of the start switch FS.

Also, since thyratron WT is non-conductive, capacitor 485 in the network AN6 is discharged and auxiliary thyratron AT7- is conductive, inverter thyratron AT 8 is nonconductive, and thyratron PT is non-conductive.

When the work M has been set between the electrodes, the star-t switch FS is closed to initiate a weld. By the closing of the start switch FS, squeeze thyratron ST and thyratron AT2 are rendered conductive.

The starting relay SR is now actuated by the squeeze thyratron ST, locking itself in at its normally open contact 412 and energizing the solenoid SV through the contact 19 and opening the valve V so that the upper electrode E1 is engaged with the work M. The pressure switch PS now starts to close. At the same time, the auxiliary relay AR is energized through the lower contact 21 of the starting relay SR and through the contact 420 of the relay RWN, energizing the ratchet relay RL and causing the contact of this relay to advance one step. The circuit through the coil 23 of the reversing contactor SKI is now opened so that this contactor drops out. Its normally open contacts K1 and K2 in series with the primary P of the welding transformer W open, and its normally closed contact K6 closes, closing a circuit through the exciting coil 25 of the other contactor SKZ and energizing the latter so that the reverse circuit through the primary P is closed. The latter reversing contactor SKZ is interlocked with SKI through its now open lower contact K which opens the energizing circuit of the coil 23 of SKI.

The actuation of the reversing contactors SKI and 8K2 takes place at the beginning of the operation of the apparatus before current flows through the welding 14 transformer W so that there is no arcing at the contacts. It is to be noted, however, that this operation does consume a portion of the squeeze interval and thus reduces the speed of operation of the complete apparatus.

When thyratron AT2 conducts, it charges the capacitor 307 of the network AN2, rendering the third auxiliary thyratrons AT3 non-conductive.

When the third thyratron AT3 becomes non-conductive, the capacitor 243 in the squeeze network SN discharges, and after an interval predetermined by the setting of the resistor 245', the potential of the capacitor 2&3 reaches a magnitude at which the auxiliary thyratron AT4 may conduct and, once the pressure switch PS in its anode circuit closes, it does conduct.

When thyratron AT4 conducts, it performs three functions. First, it charges capacitor 343 of network AN3 to render ATS non-conductive, and when ATS stops conducting, capacitor 355 of network AN4 discharges, permitting the weld thyratron WT to conduct. Second, by rendering ATS non-conductive, it also starts the timing out of the weld network. Third, it charges capacitor 4tl5 of network AN6, rendering thyratron AT7 non-conductive and starting the timing out of the post-heat network PN. So long as the post-heat network PN times out, thyratron PT remains non-conductive. This network PN is so set that it times out before the weld network WN.

The Sequence Timer (FIG. 1A) is supplied from the auxiliary buses ALT and AL2 which are supplied by the same phase buses L2 and L3 as supply the leading network N1. The weld thyratron WT then conducts in phase with the potential of the conductors L2 and L3. Through the weld thyratron WT, current is now supplied to the resistor-capacitor network 1N1 in the control circuit of the common thyratron 3NT1 of the leading heat-control network N1, conditioning this thyratron and the thyratrons 1NT1 and 2NT1 to become conductive. The thyratrons 1NT1 and 3NT1 and 2NT1 and 3NT1 are now rendered conductive at instants predetermined by the setting of the weld rheostat PHWl during successive opposite half periods of the buses L2 and L3.

For the purpose of this explanation, it may be assumed that the thyratron TNTI on the left is the first to become conductive. Energizing potential is then supplied simultaneously through the secondaries 1801 and 5301 of the output transformer 1TO1 in circuit with this thyratron to the firing thyratrons of two of the ignitrons 1-1 and 1-5 connected in series with the primary P of the welding transformer W, and current flows upwardly through the contacts K3 and K4, the primary P and the two ignitrons 1-1 and 1-5.

It is to be noted that both ignitrons I-1 and 1-5 must be rendered conductive substantially simultaneously for proper operation of the apparatus. The heat-control network N1 conceived to accomplish this purpose among others is thus an important aspect of our invention.

The conduction of current through the output resistor R01 in series with the common thyratron 3NT1 conditions the thyratron 3NT2 and the thyratrons 1NT2 and 2NT2 in series with it to conduct, and one or the other of this latter set of thyratrons is rendered conductive at an instant predetermined by the setting of the rheostat PHWZ. This rheostat is so set that one set of the thyratrons, which may be assumed to be 2NT2 and 3NT2, conducts approximately /6 of a period of the supply after the thyratrons 1NT1 and 3NT1 conduct. At this time, the upper bus L3 has become more negative than the center bus L2 with respect to the bus L1, and ignitron 1-6 is subjected to higher potential from the source (L3, L1) than ignitron I5 (L2, L1). Potential is now supplied through the lower secondary 6502 of the output transformer 2TO2 of the following network N2 in the control circuit of the firing thyratron FT6 associated with an ig- 

1. APPARATUS FOR CONTROLLING THE POLARITY OF CURRENT FLOW FROM A SUPPLY THROUGH A LOAD TO WHICH CURRENT PULSES OF OPPOSITE POLARITY ARE SUPPLIED IN SUCCESSION, COMPRISING, IN COMBINATION, REVERSING MEANS INTERPOSED BETWEEN SAID SUPPLY AND SAID LOAD, SAID REVERSING MEANS BEING OPERABLE FROM ONE POSITION IN WHICH IT PERMITS CURRENT OF ONE POLARITY OF FLOW THROUGH SAID LOAD TO ANOTHER POSITION IN WHICH IT PERMITS CURRENT OF THE OPPOSITE POLARITY OF FLOW THROUGH SAID FLOW, MEANS, INCLUDING A LATCHING RELAY, RESPONSIVE TO A FIRST EVENT FOR CONDITIONING SAID REVERSING MEANS TO BE ACTUATED FROM SAID ONE POSITION TO THE SAID OTHER POSITION, AND MEANS RESPONSIVE TO A SECOND EVENT FOR ACTUATING SAID REVERSING MEANS, FROM SAID FIRST POSITION TO SAID SECOND POSITION, AFTER IT HAS BEEN SO CONDITIONED. 